The invention relates to a hydraulic control system for a multi-position valve, the valve slide of which valve has at least one working position which can be retained by a catch element controlled by fluid pressure
Such hydraulic control systems are increasingly used for multi-position valves which control operating pressures in mobile machines By the use of such a control system, when the system pressure is below a prescribed level, the multi-position valve is held in a defined working position by the catch, but at a system pressure higher than the prescribed level, the valve can be moved out of that working position.
Such operation is conventionally achieved by means of a catch element which is acted upon by fluid pressure in a control pressure chamber such that the control pressure in the control pressure chamber acts in such a direction tending to disengage the catch means by acting against the force of a catch-engaging spring. The control lie is supplied from the system pressure, via a pressure regulating valve, so that when a prescribed system pressure is exceeded, the pressure regulating valve delivers this system pressure to the control pressure chamber and disengages the catch by acting against the force of the catch-engaging spring.
However, such a hydraulic control system is unable to also disengage the catch action in the case where a pump pressure or load pressure in the hydraulic system falls below the prescribed value. The lack of such a capability results in safety problems in the operation of the mobile machine, attributable to the fact that with customary hydraulic control systems for such multi-position valves the valve must, during start-up conditions, be moved manually into a neutral position, in order to prevent system pressure from being delivered to an operating device. If the operator forgets to do this, a dangerous situation can result.
Accordingly, an underlying object of the invention is to devise a hydraulic control system which has a simple structure and solves the above-mentioned safety problem when the system pressure is raised during start-up, and which system retains the feature whereby when the pump pressure exceeds a prescribed value the catch means are disengaged.